Estimates of phenotypic and genetic parameters and responses to selection in growth traits in three beef cattle breeds raised under ranch conditions in Botswana

Abstract

The objective of this study was to estimate phenotypic and genetic parameters and responses to selection in growth traits in the three beef cattle breeds raised under ranch conditions in Botswana. Data on birth (BW), weaning at 7 month (WW) and 18 month (18MW) weights were available for Brahman (n=841), Bonsmara (n=926) and Tuli (n=403) breeds collected from 1997 to 2008 inclusive by the Department of Agricultural Research, Botswana. Genetic and phenotypic parameters were estimated by fitting a multi-trait individual animal model in ASReml Programme and all significantly (p<0.05) fixed effects affecting performance in each trait for each breed. Repeatability, maternal and permanent environmental effects were also estimated for each trait per breed. Breeding values were estimated as by-product of Best Linear Unbiased Prediction (BLUP) from each ASREML analysis, for each breed. Generally, sex, age of dam, month of birth and year of birth significantly affected performance of growth traits in all three breeds. BW did not differ significantly between breeds, ranging from 26.±0.97kg to 32.1±2.29kg for Tuli and Bonsmara, respectively with Brahman being intermediate. WW was significantly higher in Bonsmara (215±10.9kg) than Tuli (144±5.00kg), while 18MW was significantly higher in Bonsmara (323±15.0kg) and Brahman (303±18.4kg) than Tuli (221±6.62kg) breed. Although, heritability estimates generally did not differ significantly between breeds, these were highest in Bonsmara (0.36 ± 0.12, 0.69 ± 0.08 and 0.64 ± 0.08) and lowest in Tuli (0.21 ± 0.11, 0.36 ± 0.12 and 0.21 ± 0.12) with Brahman being intermediate 0.57 ± 0.11, 0.53 ± 0.10 and 0.45 ± 0.10) for BW, WW and 18MW, respectively. Phenotypic correlations were generally low to moderate (0.20-0.60) while genetic correlations were predominantly moderate to high (0.60-0.90) between traits in each breed. Generally, repeatability estimates were predominantly moderate to high (0.60-0.90) for all traits in the three breeds. Estimates of maternal and permanent environmental effects were negligible (>1x10^-6) for each trait in all breeds. To maximise response rates trait breed specific parameters should be used for selection. There were no responses to selection in all the three growth traits across the three breeds indicating an ineffective selection index used to identify parental animals. BLUP breeding values should be used for selection especially for WW and 18MW where rapid response rates are essential to improve overall productivity.